/Engine/Plugins/Media/PixelStreaming/Source/PixelStreamingInput/Public/IPixelStreamingInputHandler.h
Set the target screen size for this streamer. This is used to when the streamer doesn't have a singular target window / viewport and as such we just use the manual scale.
void SetTargetScreenSize ( [TWeakPtr](API\Runtime\Core\Templates\TWeakPtr)< FIntPoint > InTargetScreenSize )